A newly declassified document reveals how close the United States came to accidentally detonating a nuclear bomb on North Carolina in 1961.

On January 23, 1961, a B-52 Stratofortress bomber patrolled the night skies over the Atlantic Ocean. It was three days after the presidential inauguration of John F. Kennedy, and with the Cold War in a full freeze, American bombers such as this one carrying a pair of 3.8-megaton Mark 39 hydrogen bombs were kept airborne at all times to defend the country. Many hours had passed since the B-52 took off from Seymour Johnson Air Force Base near Goldsboro, North Carolina, when something suddenly went wrong on the routine Strategic Air Command training mission.

Fuel started to gush out of a leak in the plane. Nineteen tons of fuel were lost in just two minutes. As the pilot attempted to limp back to Goldsboro, the right wing suddenly sheered from the plane. The bomber plunged into a tailspin and began to break up. Six of the eight crewmen ejected. As the plane spiraled to earth, the bombs, each of which were 260 times more powerful than the nuclear weapon dropped on Hiroshima, broke loose and plunged to the ground as well. Five of the men who were ejected parachuted to safety; the other three crew members were killed in the crash.

When responders arrived on the crash scene 15 miles from Goldsboro, they discovered one of the nuclear weapons had landed in a field with its deployed parachute tangled in the branches of a tree. The second bomb had anything but a soft landing. It became entombed after striking the ground at nearly 700 miles per hour.

While the fact that the crippled B-25 was carrying two nuclear weapons was widely reported—“Jet Carrying A-Weapons Crashes” blared a banner headline in the Greensboro Record—the military kept secret just how close the accident came to causing a nuclear catastrophe. Although the Air Force at the time reported that there was no danger of a nuclear explosion, a newly declassified document obtained under the Freedom of Information Act by author and investigative journalist Eric Schlosser as part of the research for his new book, “Command and Control,” reveals otherwise.

First published by the Guardian last Friday, the secret two-page document was written in 1969 by Parker F. Jones, the supervisor of the nuclear weapons safety department at Sandia National Laboratories. In a nod to the 1964 Stanley Kubrick film “Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb,” Jones wryly entitled his memo “Goldsboro Revisited or: How I Learned to Mistrust the H-Bomb.” While the nuclear devices that fell near Goldsboro were equipped with safety devices to prevent accidental explosions, much as revolvers have safety catches, Jones reported that three of the four safety mechanisms in the bomb that had deployed its parachute had become unlocked during its plunge to the ground. Two were rendered ineffective by the breakup of the aircraft, and a third was set off by the fall. Fortunately, the last failsafe, a low-voltage switch, worked. According to the Guardian, “When the bomb hit the ground, a firing signal was sent to the nuclear core of the device, and it was only that final, highly vulnerable switch that averted calamity.”

“One simple, dynamo-technology, low voltage switch stood between the United States and a major catastrophe,” Jones wrote. The safety supervisor also asserted that had the mid-air breakup of the B-52 caused an electrical short to the switch, “a postulate that seems credible,” it could have resulted in a nuclear explosion. Jones concluded that “The Mk 39 Mod 2 bomb did not possess adequate safety for the airborne alert role in the B-52” and that the devices designed to prevent an accidental detonation were “not complex enough.” The near-disaster resulted in more stringent safeguards being placed on America’s nuclear arsenal.

Had one of the bombs involved in the 1961 crash detonated, nuclear experts estimate that the blast would have instantly killed everything within an 8.5-mile radius. Lethal radiation fallout could have traveled up the Atlantic seaboard and stretched as far north as New York. As Jones understatedly wrote in his secret document, “It would have been bad news—in spades.”